Fix self-capturing-lambda memory leaks in context menus and ColorPickerDialog

[dovvnloading]

Summary

An adversarial bug-scan workflow found the same lambda-GC-cycle pattern already fixed for worker-owning nodes (PR #93) in an unrelated feature area: right-click context menus and the frame/container/note "change color" popup.

Root cause, identical to #93: a Python lambda closing over self, connected to a Qt signal on a widget owned by self (a QAction/QPushButton child of the menu/dialog). PySide6's GC does not reclaim this shape — a bound-method connection to the same signal is reclaimed fine, a lambda is not — so every menu/dialog instance ever constructed leaked forever, for the rest of the process.

Found and fixed in four places

• ChatNodeContextMenu: the 5 export-format actions, the per-docked-node "reveal" actions, and the per-chart-type actions all used lambda ...: self.something(...). Also pinned the ChatNode itself alive forever via self.node, on every right-click. (The reveal/chart-type lambdas weren't in the original report — found by grepping this file for the full pattern rather than trusting only the cited line numbers.)
• CodeNodeContextMenu / DocumentNodeContextMenu: the same 5-action export-menu shape.
• ColorPickerDialog: the "reset to default" button and every palette swatch button. Notably not limited to custom Signals or worker threads — a stock QPushButton.clicked calling a plain method (no Signal.emit involved) forms the identical GC-invisible cycle.

Fix

Moved the per-instance variant onto the widget itself (QAction.setData() / QWidget.setProperty()) and connected every action/button to one shared bound-method dispatcher that reads self.sender().data()/.property(...) — removes the self-capturing closure without changing behavior.

Test plan

• Verified empirically for all four (real menu/dialog + real canvas node, no mocks) that both the widget and, for the three context menus, the node it stores now collect cleanly via weakref + gc.collect()
• New tests confirm every export/reveal/chart/swatch action still dispatches to the correct variant
• Full suite: 1730 passed, zero regressions

Follow-up to the recent adversarial bug-scan series (#91–#95).